That's true in many ways and partly explains why Indeed.com (aggregation) is the largest source of online hires. The question of do 'niche job boards' have a future is interesting and I would argue yes (in the same way specialty stores still have a future).
I do see the value in true "niche" boards based around a community (HN, StackOverflow, a subreddit, whatever) since it's a way of people within the confines of that community to connect with one another. This board doesn't even seem to offer that, though; It's just based around one particular hard attribute a job listing could have. (Not that I'm trying to contradict anything you've said.)
